Transparentized paper has been used for a long period of time in various graphic applications. Such papers have been used for manual drafting operations, graphing, chart making, engineering reproduction, photocopying and tracing. The need for transparentized drawing media is increasing as a result of new technologies such as computer-aided drafting, electrostatic-type engineering copiers and for plotters including laser plotters. As used herein, the term "transparentized paper" refers to paper rendered at least partially transparent by chemical treatment. The term "parchmentized" is also employed to refer to drawing media of this type. Drafting vellum is a high-grade paper generally manufactured with a high (often 100%) content of cotton fiber and transparentized drafting vellum is widely used for engineering, architectural and other technical drawings.
For more than 40 years, the general practice in the industry has been to transparentize paper stock by impregnating it with a transparentizing medium dissolved or dispersed in a solvent. The medium is selected to have an index of refraction approximating that of cellulose fibers and serves to fill air spaces between the fibers thereby increasing the degree of transparency of the sheet.
The amount of transparentizing medium employed must be strictly controlled in order to produce a commercially acceptable transparentized paper having consistent properties. The transparentizing medium generally comprises hydrocarbons or synthetic resins and if too much is impregnated into the paper, the paper will be overly transparent, have a greasy feel and repel ink or blur markings made thereupon. If too little medium is impregnated into the paper, then it will not have the degree of transparency its use requires. In order to precisely and consistently control the amount of medium impregnated into the paper, the industry has found it necessary to dissolve the transparentizing medium in solvent and apply it in a fairly dilute form.
Use of solvent in transparentizing operations creates problems both in the process and in the final product. The solvents employed are organic solvents and must have fairly high volatility to enable their removal. Typical solvents include aromatic such as toluene or xylene, aliphatics such as petroleum ether or kerosene as well as ketones, esters and the like. These solvents are highly flammable and their use entails specialized blast-proof coating lines and high efficiency ventilation systems. Removal of solvent residue requires that the coated paper be passed through a drying line, and this step consumes significant amounts of energy and time. Furthermore, the drying lines are generally fairly long. Removal of solvent is a time/temperature process and the upper limit on temperature is the flash point of the solvent or the temperature at which the paper is damaged. Obviously, eliminating solvent would simplify the transparentizing process and apparatus and could enhance the speed at which the product is produced.
In addition to the foregoing problems of removal, the solvents present health hazards to employees and necessitate specialized handling procedures and compliance with various governmentally mandated regulations. Solvent residues must be recovered and such recovery necessitates the use of expensive, energy intensive equipment and processes.
The use of solvent based transparentizing media also creates problems with the finished product. The heat used to remove solvent from the paper also removes water from the cellulose fibers, changing the dimensions of the paper and making it more brittle. This water must be replaced; and consequently, humidifying steps involving the use of steam chambers or long exposure to ambient humidity must be implemented, further increasing the complexity and expense of the process.
Even though rigorous drying procedures are generally implemented, the finished product includes residual solvent which slowly volatilizes. This solvent residue gives an objectionable odor to the transparentized paper and can present an actual or perceived health hazards to end users. Furthermore, residual solvent can damage any apparatus which employ such transparentized papers such as copying, folding or plotting machines or any other apparatus needed to further utilize the product. Solvent-based transparentizing processes are disclosed, for example, in U.S. Pat. Nos. 4,137,046 and 3,370,949.
Aqueous-based transparentized materials have been employed in an attempt to eliminate problems associated with organic solvents. The aqueous materials employ starch, for example, as a transparentizing medium but it has been found that the products produced thereby are less than satisfactory. Papers transparentized by the aqueous process generally have a sticky or greasy feel; or they are quite hygroscopic and manifest an uneven transparency. The water from aqueous processes can detrimentally affect the properties of the paper; for example, it can cause shrinkage or poor surface quality. Paper transparentized by this process does not meet the U.S. government standard for tracing paper (Federal Specification "Paper, Tracing, UU-P-561H" published Dec. 12, 1972), and such materials have only limited commercial utility.
Another approach to eliminating solvent has been to coat a froth of transparentizing medium and air onto a web of paper in an attempt to minimize the amount of medium deposited. This process is difficult to control and has limited commercial use and it is desirable to have a simpler, more reliable alternative. Attempts to make transparentized paper have also involved the high-pressure impregnation of paper with a molten thermoplastic resin through the use of heated rollers. This process is expensive to implement and difficult to control. The product produced thereby often has an uneven transparency and is changed in its dimensions. Such resin impregnation often renders vellum papers too brittle for most purposes.
It will thus be appreciated that there is a need for a simple, solventless method of transparentizing paper on a consistent basis so that a uniform, high quality, commercially acceptable product results. A solventless process will improve the efficiency, cost and safety of the coating operation and will eliminate solvent residue, thereby enhancing the quality of the final product. Additionally, a solventless process eliminates the need for a drying step thereby maintaining the dimensional and surface qualities of the paper intact while effecting cost savings in terms of equipment, space, time and energy.
The simple expedient of immersing a sheet of paper in an appropriate solvent-free medium is unsatisfactory since the paper, (particularly thin vellum), quickly saturates and becomes useless for most graphic applications. U.S. Pat. No. 2,759,849 discloses a process for transparentizing paper-based photographic prints, which involves immersing the prints in a near-boiling mixture of mineral oil, paraffin and surfactants. This process totally saturates the paper, giving it a greasy feel and causing it to repel inks; and therefore, is of severely limited utility. There is, accordingly, a further need for a transparentizing process which in addition to being solvent-free,, produces a transparentized paper which is not totally saturated with transparentizing medium.
According to the present invention, there is provided a solvent-free method for the preparation of transparentized papers. The method of the present invention provides for precise control of the amount of transparentizing medium applied to the paper thereby providing control of the quality of the final product. These and other advantages of the present invention will be readily apparent from the drawings, discussion, description and claims which follow.